The fact that ascomycetes have asexual stages has been well-known since the work of the Tulasne brothers in the middle of last century (Weresub & Pirozynski, 1979). Most fungi producing ascospores also form mitotic spores at one or other stage of their life cycle. The meotic and mitotic spores frequently have different functions: the mitotic spores are often quickly produced with relatively little energy consumption, and are used to colonise new substrates. Ascospores more frequently have a dispersal or survival function, though it is difficult to generalize. A further function of mitotic spores is as gametes allowing the production of a diploid stage in the life cycle (which is rarely prolonged), in which instance they are termed spermatia.
Many ascomycetes rarely produce their sexual stages, at least in conditions where they are likely to be encountered by industrial mycologists. In some, only the mitotic morphs are produced in culture, while in others the fungus is rarely in practice allowed to develop fully. A large number of ascomycetes apparently never produce ascospores; they are then referred to as deuteromycetes. At one time, it was considered that the lack of a sexual stage was due to imperfections in observation or cultural manipulation. It is now becoming increasingly clear that many ascomycetous fungi genuinely lack an ascospore-producing stage, gene exchange taking place through recombination only, or through the parasexual cycle.
The necessity of considering fungi as single organisms rather than a collection of “perfect” and “imperfect” forms has resulted in new terms for the sexual and asexual stages (Hennebert & Weresub, 1979). These were immediately accepted by the mycological community and are now almost universally adopted. Thus, the “perfect” or sexual stage of a fungus is termed the teleomorph, the “imperfect” or asexual stage the anamorph, and perhaps most importantly the term holomorph is applied to the fungus in its totality.
In recognition of these links, where possible the fungi treated in this book are treated as holomorphs, using their teleomorph name where available. It is appreciated that in many fungi the anamorph is the more prominent of the stages, and indeed anamorph names are widely employed without reference to the teleomorph. At present, rules of nomenclature state that it is preferable to use the teleomorph name (which is automatically applied to the fungi in all its forms) unless anamorphic features are specifically being referred to. For those fungi without known teleomorphs, the anamorph name is de facto that of the holomorph. In these cases, the name of the anamorph must of course be retained, but where there is evidence of relationships between such anamorphic fungi and those with teleomorphs, this is recognized in the order of treatment in the descriptive texts below.
The application of different names to morphologically distinct but biologically inseparable morphs of fungi is an unfortunate, though understandable, historical accident. In time, this matter must be satisfactorily resolved so that each fungal species has a single name, but the effects of the upheaval this would undoubtedly cause must be carefully assessed.
In a small number of cases, no clear links with taxonomic groups are apparent from studies of anamorphic fungi. There is therefore a short section at the end of this chapter dealing with deuteromycetes of unknown affinity. Indeed, some of these (e.g. Moniliella and Wallemia) may well be more closely related to the basidiomycetes than the ascomycetes. They are nevertheless retained with the other deuteromycetes for practical reasons.
Most of these fungi are morphologically very simple, making assessments of their relationships difficult. Some other groups of simple deuteromycetes have clear links to more than one ascomycete group. Examples include Paecilomyces, which has links with both the Eurotiales and Clavicipitales. Its treatment is therefore divided, in recognition of its diverse relationships. The number of genera of deuteromycetes which are defined purely for morphological convenience remains regrettably large, but these obviously unnatural groups are dwindling, as more research is carried out. The new techniques of DNA analysis will be of increasing value in this task.
The ability to identify organisms accurately and to place them within a classification is of primary importance. Not to know the identity of a fungus is to cut oneself off from the whole body of literature concerning it, not only ignoring useful taxonomic information but also all previous knowledge of any of its attributes. The name of a fungus is the key to the sum total of all previous research concerning it, and it is a vital communication tool ennabling the meaningful sharing of information. Misidentifications may be very costly if they result in the misdirection of screening programmes (see below). Being able to identify fungi also ensures that contaminations of industrial processes are quickly recognized, minimizing both the disruption to production and the commercial risks of unknowingly selling contaminated materials.
Knowledge of the relationships of a fungus is also vital for mycologists from any discipline, especially those from the various branches of industry. Related fungi are likely to share particular attributes. This not only means that they look similar, but will probably share similar metabolic pathways or degrade materials in similar ways. Thus, not only can one predict with a fair degree of accuracy attributes of fungi from knowledge of their close relatives, but one may use knowledge of relationships to identify alternative species or strains which may have a more useful cocktail of properties. To take a very well-known example, the use of penicillin antibiotics on a wide scale would have been impossible without a screening programme to identify strains from the original and related species with optimum growth properties and maximal metabolite production (Raper et al., 1944). Few industrial processes involving fungi would be economically viable without knowledge of relationships ennabling the selection of the best-suited strains for environmental conditions which are often exacting in their nature. As knowledge of the relationships of fungi improves, inevitably some changes in their names occur. This is explained below.
The classification of ascomycetes has changed almost out of recognition since the previous edition of this book. This has been for several reasons. Firstly, the inclusion of data on anamorphs of ascomycetes has in many cases stabilized classifipations, and in many others ennabled the true systematic position of fungi within the Ascomycotina to be understood. Secondly, the previous almost total reliance on a small suite of characters defining major groups within the Ascomycotina has been largely replaced by assessments of the totality of exhibited characteristics (Cannon & Minter, 1992). Thirdly, lichenized ascomycetes have been integrated into the general arrangement of Ascomycotina. While most lichens have no industrial importance at present, there is increasing interest in their production of novel metabolites in pharmaceutical circles.
These changes mean that the old system of separating ascomycetes into a small number of groups based on the type of ascoma (fruit body) has been replaced by recognition of a series of orders. The first comprehensive classifications to accept these new ideas were those of Eriksson (1981, 1982) and Hawksworth et al. (1983), which included 39 and 37 orders respectively. The latest version of these classifications, taking account of recent research, suggests that there are 46 orders, and this number will probably gradually increase as further work is done. These systems have been widely accepted, although the work of others (e.g. Barr, 1983, 1987, 1990) should also be considered. In time, relationships between orders will be more widely studied, but for the present lack of knowledge precludes most speculation. The new systems are of course not completely novel, and much of the new is based closely on the old. Many of the families of ascomycetes have been retained in the new classifications, which should provide the non-specialist with reference points.
Although these changes may appear difficult to comprehend, the value in taxonomic terms of the new classifications has been immeasurably increased. When one considers that individual strains of some fungi could be placed in different basic groups of the Ascomycotina depending on the conditions in which they were grown, the artificiality and thus in most circumstances the uselessness of the old-style classifications are amply demonstrated.
For most users of this book, the intricacies of classifications of ascomycetes at the order level and above are not of direct concern. Nevertheless, the arrangement employed in this volume will prove useful, giving a much better impression of the true relationships of each fungus covered than would be the case with a traditional classification.
Nomenclature and the need for name changes
As has been explained above, the name of a fungus is the key to all the information concerning it. Clearly, in most instances something more than simple strain designation is needed to communicate information about fungi. Even if classifications do not reflect perfectly the relationships and evolutionary history of groups of fungi, whether these are at specific, generic or other level, their names are an essential means of referral.
The system of binomial nomenclature (i.e. species having a generic name and a specific epithet) was devised by Linnaeus (1753). It had several immediate advantages over the previous practice- of giving species phrase-names, which were essentially short descriptions. Each species could be referred to using two words rather than a paragraph, and as well as providing a unique means of citation, the name gave an indication of the its relationships, and often of its appearance. Thus, calling a fungus Aspergillus niger rather than “Fred” or “Brenda” suggests even to those with a basic knowledge of mycology a hyphomycetous fungus with conidiophores which are swollen at the apex, which produces chains of conidia and which probably has specific chemical and physiological properties. For those with a knowledge of Latin, the usual language of communication among scholars at the time of Linnaeus, one receives further information, that the fungus is black (Latin: niger) , and that it has the appearance of a mop for distributing holy water (Latin: aspergillum). Descriptive names are still of value in many instances, even if the last piece of information is of limited value to most today! Names are also used to commemorate the work of researchers or benefactors, or the place where they were found; thus Aspergillus chevalieri is named after the French mycologist F.F. Chevallier (1796-1840), and Aspergillus amstelodami was originally discovered in Amsterdam.
The name of the species is linked to the group of fungi itself through the citation of type material – a specific collection studied by the original author of the name. It is clearly important that this be preserved for future study, in the event that the original description provided becomes inadequate. The current rules of nomenclature state that the type material must be dried, although the preservation of cultures derived from the type material is strongly recommended where this is possible. For some groups of fungi, especially the yeasts, dried type material has been considered by some to be of little use, but with the advent of DNA analysis using PCR techniques, dried material of any microorganism is of value.
It is inevitable that from time to time, fungi referable to the same species receive different names. This is sometimes due to poor work by taxonomists, providing inadequate descriptions so that the fungus is not recognizable to others, or through recognizing spurious differences between individuals as taxonomically significant. Much of the duplication is due simply to the enormous body of literature which must be searched through to find previously-published names which may be applied to apparently novel species. This is an onerous task even for those with access to good libraries and the relevant indexes. When it becomes clear that there are two or more competing names, convention has it that the first-published name has priority. This is an equable arrangement analagous to patents in industry. Nevertheless, if a well-known name is threatened by the discovery of an earlier-published one, there is a facility under tightly-defined conditions to allow continued use of the familiar name.
Name changes for the reasons outlined in the paragraph above are regrettable, and should clearly be kept to a minimum. There are other changes which though inconvenient in the short term, have clear benefits to all mycologists, whether they work in systematics or in applied fields. These are changes due to improvements in classifications, and are inevitable given that names reflect relationships. These changes may be necessary when species are combined on the realization that they cannot properly be distinguished, or where they are subdivided when distinct groups within them are recognized. Changes may also be necessary when the relationships of a species inherent in the name are found to be false; thus the kerosene fungus previously named as Cladosporium resinae was found not to be closely related to other Cladosporium species, so its name was changed to Hormoconis resinae.
To refuse to recognize such changes is to fossilize taxonomy. Despite the feelings of some that this would be beneficial, the recognition of species and assessment of their relationship is vital to all branches of mycology. At the present, most species are defined primarily on morphological grounds, but they also show differences in such industrially useful features as enzyme activity and metabolite production. To follow on from the example at the end of the paragraph above, to search for other fungi assimilating hydrocarbons among Cladosporium species would almost certainly be an expensive waste of time.
Taxonomists are well aware of the frustration felt by some towards name changes in fungi. As explained above, some changes are entirely beneficial and must not be suppressed. Name changes simply for reasons reasons of priority must on the other hand be minimized. At present, names may be conserved to protect them from competing names. This process is inevitably rather cumbersome, and to try to speed up the process, an initiative is under way at present to produce lists of names in current use (Hawksworth, 1991b). These would then be protected against earlier competing names, contributing towards stability of nomenclature. It remains to be seen whether this enterprise is accepted by the mycological community at large, and it must be stressed that only name changes for nomenclatural reasons would be affected, a relatively small proportion of the total.
Another important duty of taxonomists is to make their work widely available. One means of doing this is a series of papers detailing name changes in fungi of industrial, microbiological and medical importance (Cannon 1986a, b, 1988, 1990). Here; the reasons for name changes in fungi significant to applied mycologists are advertized and the necessity for them explained.
The responsibility for informing workers in related fields of new taxonomic work is not one-sided. It would be most inadvisable for applied mycologists not to keep abreast of the latest developments in the classification at least of the fungi in which they have a direct interest, just as they do for advances in other branches of their science.
Literature for identification
There is currently no all-embracing text for identification of ascomycetes and their anamorphs, and in view of the magnitude of the task there is never likely to be. The best source of information is the Dictionary of the Fungi; currently in its seventh edition (Hawksworth et al. , 1983) and with the eighth planned for 1993. Further invaluable sources are to be found in two abstracting journals, the Index of Fungi and the Bibliography of Systematic Mycology, the latter now having an index to fungi treated in the papers cited. Both of these are published by IMI. A useful source-book, especially for the American market, is the compendium published by Farr et al. (1989), and the checklist of all British ascomycetous fungi by Cannon et al. (1985) provides many further references.
Other useful texts include the taxonomic treatments in Ainsworth et al. (1983), which are now outdated but still very useful. The two works by Ellis & Ellis (1985, 1988) are comprehensive guides to the identification of microfungi, which are again biased towards field collections and are largely restricted to British material. Sivanesan (1984) gives a modern (though traditional) account of the bitunicate ascomycetes and their anamorphs now referred to the Dothideales. Yeasts are well catered for by Kreger-van Rij (1984) and Barnett et al. (1991), and de Hoog et al. (1987) is essential reading.
For ascomycetes (largely ignoring anamorphs), semi-obsolete but still essential reference works include von Arx & Muller (1954) and Mil Her & von Arx (1962), dealing with ascomycetes with aseptate and one-septate ascospores respectively. Dennis (1981) provides a general treatment of British ascomycetes which is also useful to those from other countries, though he deals almost exclusively with field-collected material and concentrates on members of the Pezizales and Helotiales. The two works of Barr (1987, 1990) present an individual but often useful account of many of the groups of ascomycetes. A superbly-illustrated text which concentrates on the larger, more photogenic ascomycetes was contributed by Breitenbach & Kranzlin (1984).
Essential general texts which are restricted to deuteromycetes include the two works on dematiaceous (brown) hyphomycetes by Ellis (1971, 1975), the manual on coelomycetes by Sutton (1980), and general information on the hyphomycetes in general is provided by Carmichael et al. (1980).